The present invention relates to a mass spectrometer and method of mass spectrometry. The preferred embodiment relates to a method of calibrating a dual gain ADC detector system.
U.S. Pat. No. 7,423,259 (Hidalgo) discloses a method of operating a dual gain ADC in which the signal from a mass spectrometer is split and directed to two independent amplifiers of different gains. The two amplified signals are digitized using two independent analogue to digital recording devices. The resultant data is sent to a spectral combiner which combines the data after appropriate intensity scaling such that digitized samples from the higher gain amplifier signal path with intensities which exceed the vertical dynamic range of the ADC are replaced with the corresponding digitised samples from the low gain sample path. This composite spectrum has a dynamic range greater than either individual ADC.
In order for this approach to work correctly the time sampling intervals of both the ADCs must be correctly aligned. It is known to use firmware or complex electronics to correct the phase between two ADCs in order to align the time sampling intervals prior to the signal being digitized.
However, in addition to aligning the two ADC clocks it is also necessary to align the signal itself such that each digitized point in the two signal paths corresponds to the same region of the ion signal before individual digitized points are chosen to represent the final signal. This alignment of the signal also adds significant complexity to dual ADC operation.
It will be understood by those skilled in the art that correcting the phase between two ADCs is limited to situations wherein the phase difference between the two ADC clocks is typically less than one ADC time bin e.g. <100 ps.
U.S. Pat. No. 6,567,022 (Reuveni) discloses a method of calibrating the vertical gain and offset differences of two ADCs using a test signal. The disclosed method relates to a calibration routine to compensate for the natural variation in output amplitude response of two substantially identical ADCs when digitising the same test signal. This process is performed to allow two ADCs to be interleaved successfully to produce a single output of apparent higher digitization rate. No time correction is performed, no compensation for dual signal paths with different amplification stages is described and no dynamic range enhancement is intended.
US 2010/0213361 (Micromass) discloses a dual gain ADC method for increasing the dynamic range of a Time of Flight system wherein the signal from ion arrivals at the detector is split and sent via two amplifiers of different gain and then to two separate ADCs to be digitized. The disclosed method advantageously does not require phase correcting of the ADC clocks. According to the disclosed method the data from each push is reduced to time and intensity pairs and then combined onto a time axis which is independent of the original ADC digitisation rate. This approach allows up-sampling of the combined data from the two ADCs using finer time bins than the ADC during combining. No alignment of the signal prior to digitization is required. The method disclosed in US 2010/0213361 is therefore particularly advantageous in that the detector system does not require complex phase correction electronics.
WO 2008/008867 (Mason) discloses injecting a test pulse to adjust the phase, offset or gain of the output channels of a preamplifier having two output channels. The first output channel has a gain of eight times that of the second output channel. Two digitised data streams are stitched ADC bin to ADC bin for an individual time of flight transient before summing to produce a single high dynamic range spectra. The phase difference between the two ADCs is adjusted or corrected to zero such that the bin intervals on each ADC line up and the signal falls over the same bins in both ADCs.
To line up the ADC time bins (i.e. phase correct), an onboard ADC delay is adjusted based on the measurement of the test signal. However, this onboard delay is only capable of adjusting the phase or delay by one or two digitisation bin widths i.e. approx. 100 ps and any attempts to make larger adjustments to this delay will cause phase noise and hence timing jitter in the final signal. The disclosed method therefore only allows correction of the phase difference between the two ADCs and is not suitable for addressing the problem of significantly longer time delays due to propagation delays between the two signal paths through the two amplifiers which may be substantially greater than the onboard ADC delay can cope with.
It will be apparent that the cables of the amplifier and the ADC signal paths for the two gain channels must be very carefully designed with as close to zero phase difference as possible, such that the only correction which is needed is between the two ADCs and this is a matter of only a few ps. It will understood by those skilled in the art that such design constraints result in complex and expensive ADC detector systems and associated electronics. Furthermore, if it desired to change the amplification then a whole new circuit must be designed.
A general problem with known dual ADC systems is that due to differences in cable lengths and propagation delays through the different amplifier circuits and other components etc. there is generally a difference in recorded time between the signal passing through high and low gain signal paths even though the signal originates from the same ion arrival event at the detector. This is generally a time offset and such a time offset (typically of 10-500 ns) due to propagation delay can not be corrected by phase correcting the two ADCs (which is only able to align ADCs by approx. 100 ps).
If the two signal paths are not calibrated or aligned before the signal is combined into a final spectrum then there will be resultant loss in mass resolution and or mass accuracy in the final combined data.
In addition, although the nominal theoretical gain of the amplification circuit in each signal path may be known, in practice the actual gain or gain difference may be different. If the gain ratio used in this normalization is in error there will be quantitative errors in the final combined spectrum.
In summary, conventional dual gain ADC systems are only able to align ADC clocks by phase correcting for timing differences of up to approx. 100 ps. Conventional dual gain ADC systems require that the signal path through both ADCs is substantially identical. This requirement imposes significant design constraints and also requires complex associated electronics.
It is therefore desired to provide an improved dual gain ADC detector system for a mass spectrometer.